Lecture 2 Flashcards
1
Q
What do earthquakes result from? How is energy from an earthquake released? How are they mapped and measured?
A
- result from the rupture of rocks along a fault
- energy from an earthquake is released in the form of seismic waves
- mapped according to the epicentre, the focus is located directly below the epicentre
- they are measured by seismographs and compared by magnitude
2
Q
How can earthquake magnitude be measured quantitatively?
A
- expressed as a number to one decimal place; first developed by Richter but not used anymore
- Richter scale was a measure of strength of a wave at a distance of 100 km the epicentre
-Moment Magnitude Scale (M)
- scale is determined by area ruptured along a fault, amount of movement along the fault, elasticity of the crust at the focus
- similar to Richter scale, it is a logarithmic scale
- M7 earthquake represents 10x the amount of ground motion as an M6 earthquake
3
Q
What is the strongest earthquake to ever occur?
A
- M 9.5 in Chile in 1960
- In canada, it is M8.1 in BC in 1949
- except for very large earthquakes, Richter and M scale are similar
- there are only a few M9+ earthquakes each century
4
Q
How can earthquake intensity be measured qualitatively?
A
- Modified Mercalli Intensity Scale
- based on damage to structures and the effect on people
- based on 12 categories (1: felt by few people, 2: felt by a few people at rest especially on upper floors…12: damage is total and waves are seen on ground surface)
5
Q
Where are earthquakes most common?
A
- at or near plate boundaries
- motion at plate boundaries is not usually smooth or constant
- friction along plate boundaries exerts a force (stress) on the rocks, exerting strain or deformation
- when the stress exceeds the strength of the rocks, there is a sudden movement along a fault
6
Q
Describe the movement that occurs at a fault during an earthquake
A
- starts at the focus and propogates in all directions, called seismic waves
- faults are considered seismic sources
- identifying faults is necessary to evaluate risk of an earthquake in a given area
- not all faults reach the Earth’s surface
- blind faults are located below the surface
7
Q
How are earthquakes distributed geographically?
A
- they occur in the same places over and over again
- in Vancouver island, there is satellite monitoring for land that might be uplifting (when oceanic plate sinks it is crushing continental plate and may push it up a few mm)
- many in Oklahoma from fracking
8
Q
What are the two basic types of geologic faults?
A
- strike-slip faults: displacements are horizontal, San Andres (slabs move horizontally)
- dip-slip faults: displacements are vertical
9
Q
What are the 3 main types of dip-slip faults?
A
- reverse faults: hanging wall has moved up relative to footwall inclined at an angle steeper than 45 degrees
- thrust faults: these are similar to reverse faults except the angle is 45 degrees or less
- normal faults: the hanging wall has moved down relative to the footwall
- they are comprised of footwall and hanging wall
10
Q
How is fault activity determined?
A
- active: movement during the past 11600 years
- potentially active: movement during the past 2.6 million years
- inactive: no movement during the past 2.6 million years
11
Q
What is tectonic creep?
A
- the slow movement of rock or sediment along a fracture caused by stress
- also referred to as fault creep
- can damage roads and building foundations (movement of a few cm per decade)
- look for cracks in road, bricks of home, foundation chipping, driveway cracks
- along these faults periodic sudden displacements producing minor earthquakes can also occur
12
Q
What are different kinds of seismic waves?
A
- some travel within the body of the Earth and others travel along the surface
- body waves: P waves and S waves
- P waves: primary/compressional waves, move fast with a push pull motion and can travel through solids or liquids
- S waves: secondary/shear waves, move more slowly in an up and down motion and can only travel through solids
- surface waves: seismic waves that form when P and S waves reach Earth’s surface then move along it
- surface waves move more slowly than body waves
- responsible for damage near the epicentre
13
Q
What factors determine the shaking people experience during an earthquake?
A
- magnitude
- distance to the epicentre
- focal depth
- direction of rupture
- local soil and rock types
- local engineering and construction practices
14
Q
How does earthquake shaking appear on a seismograph?
A
- seismographs record the arrival of waves to a recording station
- because P waves travel faster than S waves, they appear first on a seismogram
- earthquake shaking decreases with distance from the epicentre
15
Q
How is distance to the epicentre determined?
A
- the difference between arrival times of the first P and S waves at different locations determine the distance to the epicentre
- the distance to the epicentre is calculated at 3 different seismic stations
- a circle with radius equal to that distance is drawn around the station
- epicetnre is located where the circles intersect; this is triangulation
16
Q
How does focal depth affect shaking at the surface?
A
- seismic waves become less intense as they spread outward toward the surface
- the greater the focal depth, the less intense the shaking at the surface
- this reduction of energy is referred to as attenuation
- waves spread the shaking out
17
Q
How does direction of rupture affect shaking?
A
- earthquake energy is focused in the direction of the rupture
- this is known as directivity and contributes to increased shaking
- radiated waves are sometimes stronger in one direction along the fault
18
Q
How does local soil and rock types affect shaking?
A
- dense homogenous crust can transmit earthquake energy quickly
- seismic energy slows down in areas with heterogeneous, folded, faulted crust
- implication: earthquakes in eastern NA are felt over larger areas than those in western NA
19
Q
What is amplification?
A
- an increase in ground motion during an earthquake
- P and S waves slow as they travel through alluvial sand, gravel, clay, soil
- alluvial: deposited by water, loose unconsolidated soil
- as the waves slow, some of their energy is transferred to surface waves
- has historically enhanced damage in San Fran area earthquakes
20
Q
What is the earthquake cycle?
A
- a hypothesis that explains successive earthquakes on a fault
- based on the idea that strain drops abruptly after an earthquake and then slowly accumulates until the next earthquake
- as stress continues to increase, the deformed material will eventually rupture
21
Q
What are the stages of the earthquake cycle?
A
Inactive period
- strain produces minor earthquakes
- period of foreshocks prior to a major release of stress (does not always occur)
- foreshock: small to moderate earthquake that occurs shortly before and in the same general area as the mainshock (M4-5)
Mainshock
- mainshock occurs allowing the fault to release built up stress
- mainshock: largest earthquake in a series of associated earthquakes
- period of aftershocks with epicentres in the same general area as the mainshock
- rock is settling after big event where there is more shaking
- always aftershocks after major earthquake
- aftershock: small to moderate earthquake that occurs shortly after and in the same general area as the mainshock
- time between stages varies
22
Q
How can the amount of aftershocks be forecasted?
A
-aftershocks on given day=aftershocks on first day after/given day
23
Q
What geographic regions are at risk from earthquakes?
A
- earthquakes are not randomly distributed
- most earthquakes occur along plate boundaries: pacific ring of fire, himalaya mountains, middle east
- North American cities at high risk of earthquakes: anchorage, vancouver, victoria, seattle, portland, san francisco, los angeles, mexico city
- however, not all areas at risk of earthquakes are near plate boundaries
24
Q
Where are earthquakes located in Canada?
A
- africa was pushing into NA which created mountains
- in the north crust is rising which may account for earthquakes
25
Q
What are plate boundary earthquakes?
A
- earthquakes that occur on faults separating lithospheric plates
- strike-slip, thrust, and normal fault earthquakes
26
Q
What is a strike-slip earthquake?
A
- occur along transform faults where plates slide horizontally past one another
- they are common in California along the San Andreas fault
- the best known strike slip earthquake is the Loma Prieta earthquake that disrupted the 1989 World Series in Oakland California
27
Q
What is a thrust earthquake?
A
- these occur on faults that separate converging plates
- they are also called subduction earthquakes
- common off of coast of BC, Washington, and Oregon
- strongest on Earth and can produce tsunamis
- subduction zone is the only place M9 can be produced
28
Q
What is a normal fault earthquake?
A
- these earthquakes occur on faults associated with divergent plate boundaries
- common along the mid-atlantic ridge
- most are located under oceans and are generally smaller than M6
29
Q
What are intraplate earthquakes?
A
- earthquake on a fault in the interior of a continent, far from a plate boundary
- these earthquakes are typically smaller than plate boundary earthquakes
- however damage could be considerable due to lack of preparedness
- because of dense continental bedrock these are felt over large areas
- two relatively active intraplate zones in NA: Central Mississippi River Valley and St Lawrence River Valley
- The New Madrid earthquakes in Missouri were over M7.5 and felt over the entire continent
- recurrence interval in this area is likely several hundred years
- recurrence interval: the time between successive events
- St Louis and Memphis are at risk
30
Q
What are the primary and secondary effects of earthquakes?
A
- primary: ground shaking, surface rupture
- secondary: liquefaction, land-level change, landslides, fire, tsunamis
31
Q
What is ground rupture?
A
- displacement along faults causes cracks in the surface
- during strong earthquakes, fault scarps can be produced that extend for hundreds of kms
- fault scarp: a linear escarpment at Earth’s surface formed by movement along a fault during an earthquake
- can uproot trees, collapse buildings, and destroy bridges, tunnels and pipelines
32
Q
What is liquefaction?
A
- the transformation of water-saturated sediment from solid to liquid
- may occur during strong earthquakes when water pressure becomes high enough to suspend particles of sediment within the soil
- once the pressure decreases, the sediment compacts and regains its strength
- watery sands and slit may flow upward along fractures in the overlying solid material
- this effect can cause extensive damage
33
Q
What are landslides?
A
- ground motion produced by an earthquake can cause rock and sediment to move downslope
- a single earthquake in a mountainous area can cause thousands of landslides
34
Q
How can ground shaking cause fires?
A
35
Q
Describe the characteristics of the Haiti earthquake?
A
- poorest country in Western hemisphere for many years
- M7 earthquake in 2010
- one of the worst natural disasters in history; death toll over 160 000
- epicentre was 25km from port au prince, most buildings in city were destroyed
- occured along a transform fault
- destruction enhanced by poor construction materials and a lack of building codes
- landslides affected slums in the hillsides surrounding the city
36
Q
A
37
Q
What are natural service functions of earthquakes?
A
- faults provide pathways for the downward flow of surface water
- they can channel groundwater to surface discharge points (springs)
- new mineral resources can be found- some minerals are preferentially deposited in faults
- scenic landscapes (hills, valleys) can develop in fault zones over millions of years
38
Q
What human activities cause earthquakes?
A
- several human activities are known to trigger small to moderate earthquakes
- weight from water reservoirs produced by dams can create new faults
- injecting liquid waste deep in earth can increase pressure and cause slippage along fractures
- fracking
- testing nuclear weapons leads to explosions that may increase strain in an area
39
Q
How can earthquake hazard be minimized?
A
- earthquakes cause death and destruction because they often occur with little warning
- at present, we can forecast the likelihood that an earthquake will occur in an area, but not exactly when it will occur
- the geological surveys of Canada and the US are developing programs to reduce hazard from earthquakes
40
Q
What are the goals of earthquake hazard reduction programs?
A
- improve national seismograph networks
- develop awareness of earthquake sources
- determine earthquake potential
- predict effects of earthquakes on buildings
- communicate research to educate the public
41
Q
How did Alaska plan for earthquakes?
A
-trans-alaska oil pipeline construction was altered where it crossed the denali fault
42
Q
How is seismic risk estimated?
A
- hazard maps identify areas of risk associated with earthquake effects
- they include areas prone to liquefaction, zones of possible ground rupture, historic epicentres
43
Q
What are precursors to earthquakes?
A
- Pattern and frequency of earthquakes based on foreshocks and microearthquakes
- Land-level change: uplift or subsidence may precede earthquakes, GPS stations can recognize small changes in elevation
- Seismic gaps along faults: areas along fault that have not seen recent earthquakes may be more likely to experience one
- Physical and chemical changes: changes in groundwater levels and chemistry may occur if rocks expand prior to an earthquake
44
Q
How can earthquakes be forecasted?
A
- must be scientifically reviewed before they are released
- research projects along the San Andreas fault are aiding in understanding the conditions that occur before an earthquake
- current earthquake warning systems provide 15-30 seconds of warning and only warn of an earthquake that has already occurred
45
Q
What is the perception of the earthquake hazard?
A
- survivors of strong earthquakes often report traumatic stress
- typically one community’s experience with an earthquake has not stimulated other communities to enhance their preparedness
- earthquakes have exposed shoddy construction practices
46
Q
What are community adjustments that can be made to earthquake hazard?
A
- not possible to prevent people from living in earthquake prone areas
- critical facilities should be located as safely as possible
- buildings must be designed to withstand vibrations (retrofitting may be required)
- education is a component of preparedness (workshops, training sessions, earthquake drills)
- earthquake insurance should be available
47
Q
What personal adjustments should be made for earthquakes?
A
- most casualties result from building collapse and falling objects
- during an earthquake it is best to stay away from windows and tall furniture
- the safest locations are under desks or tables
